These studies are designed primarily to study the factors which regulate the numbers of hematopoietic stem cells. Some studies are designed to study factors which stimulate the growth of these cells and others to study the factors which inhibit their growth. Studies are being conducted in irradiated mice and in patients who receive therapeutic radiation for non-hematologic disorders. The effects of transplanted murine leukemia on kinetics of normal and leukemic stem cells are being studied.